The method described above is known (compare, for example, German patent application DE 199 48 004 A1, page 17, line 37, to page 19, line 22, or German patent DE 100 43 405 C1, column 3, paragraph [0018], and column 8, paragraph [0052], to column 9, paragraph [0057], in conjunction with column 6, paragraph [0039], to column 8, paragraph [0050]) and is widely used, for example, not only for OEM (original) finishing but also for refinish on automobile bodies.
The so-called basecoat/clearcoat method in question is used in a wet-on-wet process to produce multicoat color and/or effect paint systems, which particularly in respect of running are in need of improvement.
Pigmented aqueous basecoat materials comprising polyurethane resins as binders are known. They can be applied to different substrates and then cured physically, thermally and/or with actinic radiation to produce coatings. They preferably comprise color and/or effect pigments and are used for producing color and/or effect coatings, examples being basecoats as part of multicoat paint systems or solid-color topcoats for—for example—automobile bodies or parts thereof. The use of aqueous coating materials is preferred from environmental standpoints to the use of coating materials which are based on organic solvents.
In the finishing of automobile bodies there is a very wide variety of application defects that may occur. One frequent defect is that of running. By running are meant the sagging of coating materials, which, though they have been applied, have not yet fully dried or cured, on vertical or inclined surfaces. This sagging generally results in an unattractive and uneven appearance in the coating that results after curing. Where this run phenomenon occurs across a relatively large area, is also called “curtaining”. In general a distinction is made between running at edges and angles, and the extensive sagging of coatings on surfaces, which is also called “slipping”. The reason for the formation of running may lie in an incorrect composition or in incorrect application of the coating material. The quality of a coating material and therefore, consequently, of a coating produced using this coating material in terms of its stability with respect to running (run stability) is frequently determined on the basis of the run limit. The run limit is the term, generally, for that wet film thickness of the applied coating material above which the first runs occur following spray application of said material to a vertical, perforated metal panel.
In the art, these run phenomena pose a grave problem, since, in the context of the industrial coating of three-dimensional substrates of complex shape, and especially in the context of automotive OEM finishing, they lower operational reliability and increase the reject rate. For instance, in the finishing of automobile bodies, there is a risk of building excessively thick coats at sharp edges of the bodies on electrostatic spray application (ESTA). If the thickness of these coats exceeds the run limit of the coating material in question, the disruptive run phenomena come about in the course of processing, more particularly during drying and thermal curing.
In the past there have been many attempts to counter this problem.
For example, in the production of multicoat color and/or effect paint systems, the automakers frequently reduce the film thickness of the basecoat, but this may have a strongly adverse impact on key performance properties such as, for example, the color intensity and color depth of the multicoat paint system, and hence on the optical quality. On the part of the paint manufacturers, attempts are made to resolve the problems by adding substantial amounts of rheological assistants, rheology control additives and/or thickeners to the coating materials, particularly to the basecoat materials. Examples are the inorganic phyllosilicates such as aluminum magnesium silicates, sodium magnesium and sodium magnesium fluorine lithium phyllosilicates of the montmorillonite type, and the silicas such as Aerosils. Thickeners also include special synthetic polymers having ionic and/or associative groups such as polyacrylamides and polymethacrylamides (poly(meth)acrylamides), poly(meth)-acrylic acids, polyvinyl alcohols, polyvinylpyrrolidones, or else styrene-maleic anhydride copolymers and derivatives thereof. Likewise used are certain modified natural materials such as hydroxyethylcellulose, carboxymethylcellulose, hydroxypropylmethylcellulose, hydroxypropylcellulose or ethylhydroxyethylcellulose. A further group of thickeners used in a very wide variety of coating materials, basecoat materials being an example, are the associative thickeners. Associative thickeners are water-soluble polymers which have strongly hydrophobic groups at the chain ends or in side chains and/or whose hydrophilic chains contain hydrophobic blocks or bundles in the interior. As a consequence, these polymers possess a surfactant character and are capable of forming micelles. They are employed as thickening additives in the production of a very wide variety of aqueous systems, as for example in auto paints or industrial coatings in general, architectural paints, printing inks, and adhesive-bonding applications. Similarly to the case with surfactants, the hydrophilic regions remain in the aqueous phase, while the hydrophobic regions become housed in the particles of polymer dispersions, are adsorbed on the surface of other solid particles such as pigments and/or fillers, and/or form micelles in the aqueous phase. Besides conventional hydrogel formation in the aqueous phase, therefore, the described solid particles of a disperse system are hence also incorporated into the structuring, resulting ultimately in a homogenization of the dispersion and in a thickening effect which frequently is substantially more effective than with the aforementioned thickeners. Numerous associative thickeners are known, and some of them have distinct structural differences. Examples are the hydrophobically modified, alkalinically activated polyacrylates, the hydrophobically modified cellulose ethers, the hydrophobically modified polyacrylamides, the hydrophobically modified polyethers, and the polyurethane-based associative thickeners. The latter are composed, for example, of hydrophilic polyether segments which are capped and/or modified with at least two hydrophobic blocks on the chain ends, on the side chains and/or inside the chain. The individual hydrophilic polyether segments and hydrophobic blocks are linked primarily via urethane bonds.
Even the use of the thickeners referred to above frequently still does not lead to the desired results in terms of run stability. Hence, for example, it is a continual concern on the part of paint manufacturers to find new possibilities for producing multicoat color and/or effect paint systems whose basecoat films exhibit a high run stability. In this way, the intention, by means of high basecoat film thicknesses, for example, is to achieve outstanding color intensity and color depth in the multicoat paint system, together with a very uniform appearance. From environmental considerations, the coatings, more particularly the basecoats, are to be based on aqueous coating materials.
The problem addressed by the present invention, accordingly, was to provide a method of the type described at the outset by which multicoat color and/or effect paint systems are obtainable which even after application of a pigmented aqueous basecoat material at high film thicknesses exhibit very little running. The run stability, therefore, ought to be outstanding, and ought, furthermore, to be improved by comparison with the prior art. In this way, it ought to be possible to produce multicoat color and/or effect paint systems, especially those on automobile bodies or parts thereof, which as well as high color intensity and color depth exhibit a very uniform surface. Accordingly, therefore, the method ought to be able to be used in particular in the sector of the automobile industry, where exacting requirements are imposed on the optical profile.